Device and method for detection and/or inspection of conductive particles using high-voltage field

ABSTRACT

Particles in a fluid cause an arc at a certain electric field whose value depends on the nature of the particles (size, composition, and the like). By applying an electric field across the fluid and determining the value of the electric field at which the arc occurs, the nature of the particles can be determined. For example, if their composition is known, their size can be determined, and vice versa. The device for carrying out such testing has a first electrode with an interior opening and a second electrode having a pin through the interior opening to define a passageway for the fluid. The electric field is varied, either by varying a voltage applied between the electrodes or by forming the passageway to be tapered. Multiple such passageways can be provided, as by forming the first electrode from honeycomb-expanded metal, thus reducing pressure drop, energy consumption without decreasing sensitivity to low concentrations.

GOVERNMENTAL INTEREST

The invention described herein may be manufactured, used and licensed byor for the U.S. Government.

FIELD OF THE INVENTION

The invention is directed to an apparatus and method for detecting lowconcentrations of a smoke having a small particle size (e.g., 10microns).

DESCRIPTION OF RELATED ART

There is a need in the art to detect smoke having low concentrations anda small particle size (e.g., 10μ particles of carbon or brass).Conventional detection of such smoke has relied on optical detectionusing visible, infrared, or ultraviolet light. However, such opticaldetection is dependent on the particles' ability to scatter, absorb, orforward scatter light and have limited performance in extremely lowconcentrations of smoke.

Co-pending application Ser. No. 09/400,146, filed Sep. 12, 1999,entitled “Device and Method for Inspection and Detection of a Materialby Observing a High-Voltage Waveform Produced by that Material”, teachesdetecting low impedance fibers by providing a high voltage between twobrass screens and observing an arc when a fiber bridges the gap betweenthe screens. However, this technique is not well suited to detectingsmall particles, as the gap between the screens is far too large.Flat-plate designs used in corona-discharge systems for spectroscopy aresimilarly unsatisfactory.

SUMMARY OF THE INVENTION

It is an object of the invention to detect the presence of smoke (e.g.,particles of carbon or brass) in a fluid, particularly in air.

It is another object of the invention to detect the presence of smoke ina large volume of air.

It is a further object of the invention to detect the presence of smokewith a reduced lag time while detecting the smoke in an enclosure.

It is a further object of the invention to detect the presence of smokein a device with a small pressure drop so as to conserve power andreduce noise.

To achieve these and other objects, the present invention is directed toa detector for detecting particles in a fluid, the detector comprising:a voltage supply for supplying an adjustable voltage; a first electrodehaving at least one interior opening extending through the firstelectrode; and a second electrode comprising at least one pin extendingthrough the at least one interior opening to define at least onepassageway for the fluid; the first electrode and the second electrodebeing connected to the voltage supply so that the adjustable voltage isapplied between the first electrode and the second electrode. Thepresent invention is further directed to a detector for detectingparticles in a fluid, the detector comprising: a voltage supply forsupplying a voltage; a first electrode having an interior openingextending through the first electrode; and a second electrode comprisinga pin extending through the interior opening to define a passageway forthe fluid; the first electrode and the second electrode being connectedto the voltage supply so that the voltage is applied between the firstelectrode and the second electrode to define an electric field betweenthe first electrode and the second electrode; and at least one of theinterior opening and the pin being shaped so that the passageway istapered and the electric field varies along the length a method ofdetecting particles in a fluid, the method comprising: (a) providing adetector comprising (i) a voltage supply for supplying a voltage, (ii) afirst electrode having at least one interior opening extending throughthe first electrode, and (iii) a second electrode comprising at leastone pin extending through the at least one interior opening to define atleast one passageway for the fluid, the first electrode and the secondelectrode being connected to the voltage supply so that the voltage isapplied between the first electrode and the second electrode to producean electric field between the first electrode and the second electrode;(b) introducing the fluid into the passageway; (c) applying the voltagefrom the voltage supply to the first electrode and the second electrode;(d) varying the electric field; (e) determining a value of the electricfield at which an arc occurs in the fluid; and (f) detecting theparticles in accordance with the value of the electric field determinedin step (e).

The invention works on the principle that the discharge threshold, whichis the potential difference required for a breakdown of an electricalfield (E field) to produce an arc, is dependent on the conductivity ofthe sum of the impedances of the media between the opposing electrodesused to apply the potential difference and on the distance between theelectrodes. The conductivity is influenced by the base medium (such asair), the presence of foreign particles in the base medium (such ascarbon particles in smoke), and the size and composition of theparticles. If the base medium is uniform (as is the case with air andwater), it is the particles which cause the change in the dischargethreshold. Since the effects of the particles are determined both bytheir size and by their conductivity, particles in a stream can beanalyzed. If the material is contaminated, e.g., with water, theconductivity changes, and so does the threshold.

A design featuring a cylinder with a rod or pin in the center, both ofwhich function as electrodes, and a design featuring a honeycombarrangement of such cylinders and rods or pins allow a largecross-sectional area for air flow and thus are relatively unobtrusive tothe fluid flow while minimizing the distance between the electrodes.Thus, sensitivity is improved, while high flow rates and low pressuredrop are maintained.

The discharge threshold also depends on the particle size. If it isimportant to detect particle size as well, the E field can be set so asto detect particles above or below a cutoff size by determining whetherthe arc occurs. The E field can be varied during such detection, and thevalue at which the arc starts or stops allows a determination of theparticle size.

The limit setting of the E field is dependent on the particle size andconductivity. In a stream of unknown particles, this dependence allows alevel of filtering particle types by varying the E field.

A preferred embodiment of the invention uses highly charged conductiveprobes (electrodes) spaced apart to form an E field and senses thevoltage required to cause an arc. The voltage between the electrodes,and with it the E field, can be increased or decreased, and knowledge ofthe voltage at which arcing occurs allows a determination of the content(chemical composition of the particles) if the particle sizes andconcentrations are known. In the case of particles having a knownchemical composition, the size or concentration of the particles can bedetermined. These determinations are possible because particles of thesame size and different compositions arc at different voltages, as doparticles of different sizes and the same composition. An experiment wasconducted with cigarette smoke, in which the difference between inhaledand uninhaled smoke could be seen.

The device and method according to the present invention are highlysensitive to all kinds of conductive particles and is especiallyvaluable for small particles. The present invention is not dependent onthe particles' ability to scatter, absorb, or forward scatter light inthe IR, visible, or UV range.

Conductive particles have uses in both governmental and commercialareas. Besides detection and identification of obscurants in militaryapplications, the invention can be used to detect contaminants in wateror another fluid.

While it is contemplated that the invention will be used with an airflow, any fluid can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be set forth indetail with reference to the drawings, in which:

FIG. 1 shows a side schematic view of a detector according to a firstembodiment of the present invention;

FIG. 2 shows a head-on view of the electrodes in the detector of FIG. 1;

FIG. 3 shows a side schematic view of a detector according to amodification of the first embodiment of the present invention;

FIG. 4 shows a head-on view of the electrodes in the detector of FIG. 3;

FIG. 5 shows a flow chart of operational steps implemented with thedetector of FIG. 1 or FIG. 3;

FIG. 6 shows a side schematic view of a detector according to a secondembodiment of the present invention;

FIGS. 7 and 8 show cross-sectional views of the detector of FIG. 6;

FIG. 9 shows a side schematic view of a detector according to amodification of the second embodiment of the present invention;

FIG. 10 shows a side schematic view of a detector according to anothermodification of the second embodiment;

FIG. 11 shows a side schematic view of a detector according to stillanother modification of the second embodiment; and

FIG. 12 shows a flow chart of operational steps implemented with thedetector of FIG. 6, FIG. 9, FIG. 10, or FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a side schematic view of a detector according to a firstembodiment of the present invention. In detector 101, high-voltage (HV)supply 103, which can be any suitable HV supply and which can beadjusted through manual or automatic adjustment control 104, supplies apotential difference to two electrodes, namely, cylinder 105 havinginterior opening 106 and pin 107 disposed in interior opening 106 ofcylinder 105. Pin 107 is preferably disposed in the center of cylinder105, as shown in a head-on view in FIG. 2. Cylinder 105 and pin 107define passageway 109 between them, formed in part of interior opening106, to accommodate an air flow, which can be in the direction shown byarrow A or in the opposite direction.

FIG. 3 shows a side schematic view of a modified version of detector 101of FIG. 1. Detector 301 of FIG. 3 differs from detector 101 of FIG. 1 inthat cylinder 105 is replaced with electrode 305 having multipleinterior openings 306 therein and that multiple pins 107 are provided,each in one of multiple interior openings 306. Each combination of a pin107 and an interior opening 306 defines a passageway 309 for air flow.The arrangement of pins 107, interior openings 306 and passageways 309in electrode 305 is shown in a head-on view in FIG. 4. HV supply isconnected to electrode 305 and to each of pins 107.

The configuration shown in FIGS. 3 and 4 offers advantages in terms ofease of alignment, a small gap between electrode 305 and each of pins107, and the availability of materials. Such a configuration allows manyclose-gapped electrodes with tight tolerance in spacing to be containedin a small cross section and still to provide a large open area,reducing pressure drop at high fluid flow and thus reducing powerconsumption. Also, a lower linear velocity of fluid flow can be usedwhile maintaining a suitable flow in terms of volume per unit time.

Electrode 305 can be formed from honeycomb-expanded metal. Suchhoneycomb-expanded metals are known, and available materials offersuitable conductivity and dimensional stability. Alternatively,electrode 305 can be formed from multiple cylinders, generally likecylinder 105, held together in a suitable manner.

Pins 107 are brass brads. In modified detector 301, pins 107 are mountedon precision-drilled brass plate 311 with hole dimensions providing aforce fit. This arrangement provides a conductive, mechanically stablesupport for extremely close air gaps between pins 107 and electrode 305.

Detector 101 and detector 301 can be used to implement the series ofoperational steps shown in the flow chart of FIG. 5. These steps can beperformed either manually or automatically, as by any suitablyprogrammed computer.

The operation starts in step 501. In step 503, a fluid flow is startedin passageway 109 or passageways 309, either by providing a blower or byexposing detector 101 or 301 to wind or to fluid flow. In step 505, thehigh voltage is applied from HV supply 103. In step 507, the voltageapplied from HV supply 103 is increased or decreased by controlling HVsupply 103 through adjustment control 104, thus also increasing ordecreasing the E field, until an arc occurs. In step 509, the voltage atwhich arc occurs is determined.

Steps 511-515 involve determining an unknown characteristic of theparticles from the voltage and another known characteristic. Forexample, it is determined in step 511 whether the particles are of aknown size or of a known material. If they are of a known size, thematerial is determined in step 513. On the other hand, if they are of aknown material, the size is determined in step 515. Othercharacteristics, such as concentration, can be detected if enoughinformation about the particles is already known. The range ofcharacteristics that can be detected is especially broad if particlesize can be determined in another way, e.g., optically. Either way, theoperation ends in step 517.

The first embodiment of the invention, as disclosed in FIGS. 1-5 and thedescription thereof set forth above, has been experimentally verified towork with carbon smoke particles, brass smoke particles and cigarettesmoke. It was experimentally verified that a single particle of smokecould be detected with a sufficiently high E field and that inhaledcigarette smoke could be distinguished from uninhaled cigarette smoke.

In the first embodiment, the E field is varied by controlling the HVsupply. A second embodiment will now be set forth in detail, in whichthe E field is varied by displacement along the length of the detector.

FIG. 6 shows a side schematic view of a detector according to the secondembodiment. FIGS. 7 and 8 are cross-sectional views taken along arrowsVII—VII and VIII—VIII, respectively.

As shown in FIGS. 6-8, detector 601 includes HV supply 603 which is notrequired to be adjustable. Electrode 605 has tapered hole (interioropening) 606 therethrough to provide tapered passageway 609 for fluidflow, preferably in the direction indicated by arrow A. Slot 613 extendsfrom the outermost surface of electrode 605 to passageway 609 to allowobservation of the location at which arcing occurs. Pin 107 is disposedin tapered hole 606 to define passageway 609, preferably in the centerof electrode 605. Arcing in passageway 609 can be observed through slot613, either directly by a person or with photocell array 615.

Because of tapered passageway 609, the distance between electrode 605and pin 107 varies along the length of the detector, and with it the Efield. Thus, for a constant E field, the discharge threshold occurs at aspecific location along passageway 609. Observation of this locationthrough slot 613 allows determination of the E field and thus of thedesired characteristic of the particles.

FIG. 9 shows a side schematic view of a modified version of detector 601of FIG. 6. Detector 901 of FIG. 9 includes HV supply 603 andphotodetector array 615 like those of detector 601 of FIG. 6. However,detector 901 includes cylinder 905, whose interior opening 906 is nottapered, and tapered pin 907 to provide passageway 909 with aposition-varying gap between cylinder 905 and tapered pin 907. Cylinder905 can be like cylinder 105 of detector 101 of FIG. 1, except for slot913 to allow observation of the arc.

The location of the arc can alternatively be detected in accordance witha distributed-ground technique. This technique relies on the fact thatat the location of an arc, current flow markedly increases. Detection ofthe increased current flow allows detection of the location of the arc.Both the interior opening and the pin could be tapered.

FIG. 10 shows a side schematic view of detector 1001, which is similarto detector 601 of FIG. 6 in having HV supply 603 and pin 107. However,electrode 1005 differs from electrode 605 in not having slot 613.Interior opening 1006 and tapered passageway 1009 are thus like interioropening 606 and tapered passageway 609, except without any interruptionfor a slot.

Detector 1001 differs further from detector 601 in that HV supply 603and electrode 1005 are connected through an array of detection resistors1017 in parallel. Detection resistors 1017 change some physical propertywhen current passing through them goes over a threshold. The location ofthe arc can thus be determined in accordance with this changed physicalproperty. Alternatively, conventional resistors could be used, and thecurrent flow could be detected across each of the conventionalresistors.

The modifications shown in FIGS. 9 and 10 can be modified to formdetector 1101 of FIG. 11. Detector 1101 combines detection resistors1017 of detector 1001 with tapered pin 907 of detector 901 and cylinder105, with no slot and with non-tapered interior opening 106, of detector101. The combination of cylinder 105 and tapered pin 907 formspassageway 1109, which is like passageway 909 except for the absence ofa slot. Again, both the pin and the interior opening could be tapered.

Any of detectors 601, 901, 1001, and 1101 can be modified to havemultiple passageways, as does detector 301.

The detectors of FIGS. 6-11 can be used to implement the series ofoperational steps shown in the flow chart of FIG. 12. The operationalsteps of FIG. 12 are the same as those of FIG. 5, except that steps 507and 509 are replaced with step 1207 of determining a location at whichthe arc occurs (either by photodetector array 615 or by detectionresistors 1017) and step 1209 of determining an electric fieldcorresponding to that location.

While two preferred embodiments of the invention have been described indetail, each with variations, those skilled in the art who have reviewedthis disclosure will readily appreciate that other embodiments can bedescribed within the scope of the invention. For example, modificationsdisclosed together can be used separately, while modifications disclosedseparately can be combined. Also, any conventional analytical hardware,software, or techniques can be incorporated into setups according to theinvention. Any disclosed process can be automated. The data collectedcan be combined with electron microscope pictures, manufacturinginformation, low-voltage measurements, or any other information usefulin analysis of the material. Materials and numbers of componentsdisclosed are illustrative rather than limiting.

I claim:
 1. A method of detecting particles in a fluid, the methodcomprising: (a) providing a detector comprising (i) a voltage supply forsupplying a voltage, (ii) a first electrode having at least one interioropening extending through the first electrode, and (iii) a secondelectrode comprising at least one pin extending through the at least oneinterior opening to define at least one passageway for the fluid, thefirst electrode and the second electrode being connected to the voltagesupply so that the voltage is applied between the first electrode andthe second electrode to produce an electric field between the firstelectrode and the second electrode; (b) introducing the fluid into thepassageway; (c) applying the voltage from the voltage supply to thefirst electrode and the second electrode; (d) varying the electricfield; (e) determining a value of the electric field at which an arcoccurs in the fluid; and (f) detecting the particles in accordance withthe value of the electric field determined in step (e).
 2. A method asin claim 1, wherein: the at least one interior opening comprises aplurality of interior openings, each of the plurality of interioropenings extending through the first electrode; and the at least one pincomprises a plurality of pins, each of the plurality of pins extendingthrough one of the plurality of interior openings such that the at leastone passageway comprises a plurality of passageways.
 3. A method as inclaim 2, wherein the first electrode comprises honeycomb expanded metal.4. A method as in claim 3, wherein the second electrode furthercomprises a plate having a plurality of holes, each of the plurality ofpins being force fitted into one of the plurality of holes.
 5. A methodas in claim 2, wherein the second electrode further comprises a platehaving a plurality of holes, each of the plurality of pins being forcefitted into one of the plurality of holes.
 6. A method as in claim 1,wherein the first electrode comprises a cylinder.
 7. A method as inclaim 1, wherein: the voltage supply comprises means for adjusting thevoltage; step (d) comprises varying the voltage over time to vary theelectric field over time; and step (e) comprises determining the voltageat which the arc occurs.
 8. A method as in claim 1, wherein: at leastone of the interior opening and the pin is shaped so that the passagewayis tapered; and in step (d), the electric field varies along a length ofthe pin.
 9. A method as in claim 8, wherein the interior opening istapered.
 10. A method as in claim 8, wherein the pin is tapered.
 11. Amethod as in claim 8, wherein step (e) comprises: (i) determining alocation at which the arc occurs; and (ii) determining the value of theelectric field in accordance with the location determined in step(e)(i).
 12. A method as in claim 11, wherein: the first electrode has aslot extending to the interior opening to allow observation of thepassageway; and step (e)(i) comprises observing the arc through theslot.
 13. A method as in claim 12, wherein said step of observing isperformed with a photodetector array adjacent to the slot.
 14. A methodas in claim 13, wherein: the voltage supply is connected to the firstelectrode through an array of resistors which are disposed in parallel;and step (e)(i) comprises: (A) detecting an increased current flowacross one of the resistors; and (B) determining the location inaccordance with the increased current flow detected in step (e)(i)(A).15. A method as in claim 14, wherein: each of the resistors in the arrayis a detection resistor which changes a physical property in accordancewith a flow of current in said detection resistor; and step (e)(i)(A)comprises detecting the physical property.